The present invention relates to a current mirror circuit utilizing a resistor ratio to produce an output current which is proportional to an input current.
Current mirror circuits are some of the most fundamental building blocks of analog integrated circuit design. Typical current mirror applications include current sources used for biasing and active loads which are part of an AC signal path. Current mirrors can also be used as fixed-gain current multipliers by ratioing the emitter areas of the transistors in the mirror. Some desirable current mirror attributes include accuracy, thermal immunity, low voltage drop and maximum output compliance, and high speed. Most precision current mirrors achieve their accuracy at the sacrifice of voltage drop, output compliance and speed. Moderate thermal immunity and improved accuracy can be achieved by degenerating the mirror with resistors which can generally be matched in an integrated process better than transistors.
High current applications such as laser diode drivers can require better thermal immunity than can be achieved by simply degenerating the mirror, especially when the mirror's current ratio is other than one. Traditional current mirrors, such as the one shown in FIG. 1, are inherently thermally dependent as they rely on the base-emitter voltage of a transistor to determine the output current and the base-emitter voltage and collector current relationship of a transistor is very temperature dependent.
Conventional current mirror circuits typically are comprised of a diode-connected transistor coupled across the base-emitter of an output transistor wherein the former biases the latter. With reference to FIG. 1, there is shown a prior art current mirror circuit 10 which includes transistor 12 connected as a diode having its collector and base interconnected to input 14 while its emitter is returned to power supply rail 18. The base of transistor 12 is also connected to the base of transistor 16, the emitter of which is coupled to rail 18 while an output current is provided at output 20. An input current is supplied to the diode-connected transistor, which has its collector and base interconnected to the base of the output transistor while the emitters of the two devices are interconnected. By emitter area ratioing of the two transistors, an output current is developed at the collector of the output transistor that can be less than, greater than or equal to the input current.
A typical problem with conventional current mirror circuits of this type is related to poor thermal matching between input and output transistors. In addition, for current ratios that are not one, matching becomes very difficult in conventional circuits because identical transistors cannot be used.
Accordingly, it is an object of the present invention to provide a current mirror circuit which provides a means of implementing a current mirror with an arbitrary, but fixed, current ratio for high current and high power dissipation environments in which the load voltage is unknown and maximum output compliance is required.
It is another object of the present invention to provide a current mirror circuit which provides a means for implementing a current mirror with an arbitrary, but fixed, current ratio for high current and high power environments which require excellent thermal immunity, low voltage drop, and maximum output compliance, while achieving the maximum possible speed (bandwidth).